Contain

What does Contain Mean?

A defensive strategy in which the pass rush will
attempt to keep the quarterback in the pocket to avoid letting him
scramble for more time to pass or run with the football. Contain is
particularly important when facing quarterbacks with good running
ability, since a quarterback who breaks out of the pocket will
often have the defense out of position and can gain big yardage on
the ground. The easiest strategy behind keeping contain is to have
the defensive ends or outside linebackers bracket the quarterback
in order to keep him in the pocket, and slowly shrink the pocket
using the pass rush to cause pressure.

Sporting Charts explains Contain

Contain is most important to defenses that are
worried about a mobile quarterback doing damage with his legs. A
quarterback who gets out of the pocket is much more likely to
deliver a big pass or make a big run. By keeping contain, the
quarterback is limited in his options on a particular play. Good
defensive linemen will stay in their pass rush lanes so that they
team as a unit can contain the quarterback and keep him in the
pocket. A defensive player can lose contain if they are too
aggressive in their pass rush and get out of position.